Ignition delay or lag of fuels is well-known and is generally defined as the time interval between initial injection of a fuel into an oxidant and ignition or exothermic oxidation sufficient to sustain continued combustion. It is also well-known that different fuels have different ignition delays and that a major obstacle of multiple fuel engines is the difference in ignition delay. Theories as to the cause of ignition delay abound. One rather widely accepted theory holds that only radicals of fuel molecules oxidize, that is, ignite or combust and that ignition delay is the time interval between initial injection of the fuel molecules into an oxidizing medium such as air and the formation and oxidation of radicals in sufficient number to sustain continued combustion. Parameters believed to affect the formation of radicals are initial pressure in a combustion chamber and preheating of the fuel and/or air. Whether or not this theory is correct, empirical data clearly shows that initial pressure in a combustion chamber and preheating of the fuel and/or air affects ignition delay.
Numerous investigations have been performed on ignition delay of fuel injected into air. The majority of these investigations concentrated on the effects of initial combustion chamber pressure and initial air temperature on ignition delay since other investigations had shown preheated fuel at temperatures as high as 500 degrees Kelvin (.degree.K.) had little or no effect on ignition delay. However, when a few investigators did preheat fuel to temperatures considerably above 500.degree. K., they found significant decreases in ignition delay.
For example, H. C. Gerrish and B. E. Ayer disclosed in 1936 measured ignition delays of fuel preheated between 324.degree. K. and 672.degree. K. in a compression ignition engine of swirl chamber design and running at 1500 RPM or 9.times.10.sup.3 crankshaft angle degrees per second. At 324.degree. K., measured delays were approximately ten crankshaft degrees or 1.1.times.10.sup.-3 seconds. At 672.degree. K., measured ignition delays were approximately six crankshaft degrees or 0.67.times.10.sup.-3 seconds. Comparisonwise, the ignition delays decreased 0.43.times.10.sup.-3 seconds or 39%.
Further, V. V. Holmes et al. in 1975 disclosed the effects of fuel vaporization on combustion in a combustion chamber for a turbojet engine. Visable observations were made of flame distance from a nozzle injecting fuel at increasing preheated fuel temperatures into an open atmosphere without forced air movement. The observed visible flame distance decreased with increased fuel preheating and thereby suggested decreasing ignition delay with increased preheating. No visible distance was observed between the nozzle and flame for preheated fuel above 617.degree. K.
Still further, L. J. Spadaccini in 1976 disclosed results obtained in a steady-flow test facility in which ignition delay of various fuels was measured by varying not only the temperature of preheated air, but also by varying the temperature of preheated fuel to temperatures as high as 700.degree. K. Spadaccini's results are summarized herein in FIG. 1, plotting ignition delay, .tau., versus reciprocal air temperature.
While the above-mentioned investigations are significant and add to our knowledge of ignition delay, they fail to disclose or suggest that ignition delay can be reduced to negligible amounts such as 5.times.10.sup.-5 seconds or less. Further, they fail to disclose or suggest that ignition delays of fuels of different molecular structure, i.e., families of fuels, can be made both negligible and negligibly different.
By way of example, ignition delays in the order of 5.times.10.sup.-5 seconds in a piston engine at 1500 RPM equate to 0.45 crankshaft angle degrees compared to the six crankshaft angle degrees in the Gerrish and Ayer investigation. Further, even if the engine were operated at speeds on the order of 6,000 RPM, ignition delay would still be less than 1.8 crankshaft angle degrees.
The advantages of negligible ignition delays or substantially instantaneous combustion are many. For example, it is well-known that piston or expandable chamber engines fail to efficiently and completely combust fuel due to ignition delay. In such engines, fuel with a negligible ignition delay could be injected into the expandable chamber at the precise or optimum moment to effect combustion and useful work. In a Diesel Cycle engine, such fuel could be injected at a rate necessary to effect true constant pressure combustion, thereby preventing diesel knock which is not only annoying but also detrimental to engine structure and combustion efficiency. Families of fuels conditioned to have both negligible ignition delays and ignition delay differences make possible the conversion of virtually all combustion engines to the use of multiple fuels.